Transport elements in the above described general field are frequently used in photocopy machines, printers, or other office machines for transporting paper, cardboard, film or similar sheet goods. In this context, it is known to use embodiments of the transport element having a circular support member and a circular ring member, as well as embodiments having a circular segment-shaped support member and a similarly circular segment-shaped ring member fittingly arranged thereon, e.g. so-called D-rollers.
In a generally known embodiment of such a transport element, the support member comprises a hub for receiving an axle or a shaft, and has an outer circumferential surface provided with nubs or protrusions that project radially outwardly and that are arranged uniformly distributed around the circumference of the outer circumferential surface. These nubs have a pyramidal frustum shape, with an increasing cross-sectional area as the radial distance from the hub increases. In a manner of speaking, the pyramidal frusta are standing on their heads, i.e. are arranged with their narrower ends facing radially inwardly. Thus, each pyramidal frustum-shaped nub is connected by only a relatively small surface to the outer circumferential surface of the support member.
During the manufacturing of the known transport element, the support member is first produced from a thermosetting or a thermoplastic synthetic material by an injection molding process. Next, the support member is inserted into another injection molding tool, in which the ring member is formed by injection molding a rubber-elastic material around the support member. When the material for the ring member is in a molten flowable condition, it can completely flow around and enclose the nubs on the surface of the support member.
After the material of the ring member cures, a form-locking connection results between the support member and the ring member, due to the form of the individual nubs having cross-sections tapering radially inwardly toward the outer circumferential surface of the support member. This interconnection achieves a rotational rigidity of the ring member relative to the support member and also secures the ring member against a radially directed pulling-off thereof. Furthermore, an axial shifting of the ring member relative to the support member is prevented, since the pyramidal frustum-shaped nubs have a smaller axial extension in comparison to the axial width of the support member, both in the area of the base and also in the area of the frustum peak of the nubs.
It has become apparent as a very great disadvantage in the known transport elements, that the effective elasticity of the transport element measured in a radial direction comprises large fluctuations when evaluated around the circumferential direction along the contact surface of the ring member. In other words, different locations on the contact surface around the circumference thereof will exhibit greatly differing radially directed elasticities. These fluctuations in the elasticity result from relatively large variations or differences in the effective thickness of the ring member made of the rubber-elastic material, whereby this effective thickness is a decisive factor determining the elastic behavior of the transport element. Namely, the ring member is considerably thicker in the area between the respective nubs than in the area of the base of each respective upside-down pyramidal frustum-shaped nub. Since the elasticity of the rubber material of the ring member is generally considerably greater than the elasticity of the material of the support member and particularly the nubs thereof, the thickness of the ring member is predominantly significant for establishing the effective radial spring stiffness or spring constant of such a rollingly supported transport element.
As a result of the above described fluctuations of the effective spring constant or elasticity of the known transport element, such a transport element has unsatisfactory characteristics with regard to an exact conveying or transporting behavior. In other words, the known transport element does not satisfactorily and uniformly achieve an accurate and precise transport of the flat goods being conveyed. The requirements as to the exact positioning of the sheets or films being conveyed by such transport elements have constantly increased in the past and are also expected to continue to increase in the future. Therefore, conventional transport elements can no longer meet these requirements.
Attempts to reduce the differences in the angle-dependent effective spring constants by increasing the thickness of the ring member around the entire circumference, while maintaining the same geometry of the support member, have resulted in two disadvantages. First, the axial flexural stiffness of the transport element is reduced, especially in the area of the contact surface. Secondly, the cost of the transport element is increased due to an increased consumption of the comparatively expensive rubber-elastic material of the ring member.